1. Field of the Invention
The present invention relates to a power amplifier, and more particularly to a parallel push-pull amplifier using a complementary device, which basically operates for a B or AB-level amplification while having a common source configuration, thereby being capable of amplifying the full wave of an input signal without any distortion while obtaining a high gain at a radio frequency.
2. Description of the Prior Art
Generally, push-pull amplifiers have a configuration including a pair of symmetrically-connected active elements. Voltage signals having the same voltage level, but having a phase difference of 180.degree. therebetween are applied to respective input terminals of the active elements. Since such a push-pull amplifier operates in such a manner that non-linearities of characteristic curves of transistors, which are used for the active elements, complement each other, it achieves an improvement in linearity and an increase in distortionless maximum power. In this regard, such a push-pull amplifier is used for a power amplifier.
Typically, push-pull amplifiers have an operating point corresponding to a cut-off point (class B amplification) in order to achieve an improvement in power efficiency.
Where such push-pull amplifiers are used for a low frequency amplification, they have a connection in the form of a source follower.
Meanwhile, for a radio frequency amplification, amplifiers having a connection in the form of a common source are used.
Now, examples of the above mentioned source follower type push-pull amplifier and common source type amplifier will be described in conjunction with FIGS. 1 and 2.
FIG. 1 is a circuit diagram illustrating a conventional push-pull amplifier having a source follower configuration. In this push-pull amplifier shown in FIG. 1, when an input signal, which has a maximum voltage level and a positive polarity, is applied to the input of the amplifier, one transistor, namely, an NMOS transistor Q1, turns on, while the other transistor, namely, a PMOS transistor Q2, turns off. As a result, a positive voltage Vdd, which is a drive voltage applied to the drain of the NMOS transistor Q1, is applied to the output of the amplifier.
On the other hand, when the input signal applied to the input of the amplifier has a negative polarity, the NMOS transistor Q1 turns off, while the PMOS transistor Q2 turns on. Where an input signal, which has a minimum voltage level and a negative polarity, is applied to the input of the amplifier, the drive voltage Vdd applied to the drain of the NMOS transistor Q1 is prevented from being applied to the output of the amplifier. In this case, the potential of the ground connected to the drain of the PMOS transistor Q2 is applied to the output of the amplifier.
The above mentioned push-pull amplifier using a source follower configuration is advantageous in that it exhibits a small output resistance. In particular, where such a push-pull amplifier is used for a class B or class AB amplification, it is possible to obtain a maximum efficiency.
The push-pull amplifier of FIG. 1, which uses a source follower configuration, has a limitation in the output voltage swing range. That is, the output voltage swing range is limited between the ground potential and the drive voltage. Furthermore, the push-pull amplifier achieves a power amplification only using a current gain, namely, without using any voltage gain, because of its performance characteristics. This results in a considerable limitation in obtaining power gain. As a result, the power-push amplifier is limited to applications for a low frequency amplification because its maximum available gain is small.
For this reason, the above mentioned push-pull amplifier, which uses a source follower configuration suitable only for a low frequency amplification, is improper for a radio frequency amplification requiring an efficient use of the the characteristics of the entire device. In view of such a problem, amplifiers of a source grounding type, called a common source type, have been proposed for a radio frequency amplification.
Referring to FIG. 2, a conventional common source type amplifier is illustrated. In this amplifier, an inductor Lb having a choke function is used to hold a bias. Accordingly, a constant bias voltage Vdd is provided at the output of the amplifier. At this time, a current Idd is also output. When an AC signal is applied to the amplifier, the inductor Lb serves as an open circuit for radio frequency signals. Accordingly, the AC signal is added to the bias voltage, which is a DC signal. As a result, the current at the output of the amplifier may vary in a range from 0 to 2 Idd. In this case, the output voltage may also vary in a range from 0 to 2 Vdd. Therefore, there is an advantage in obtaining high power.
Although the amplifier having the above mentioned common source configuration is advantageous in that a high gain can be obtained at a radio frequency, as compared to that of the source follower configuration, it involves a severe distortion of output waves when it is used for a class B or class AB amplification. This is because the amplifier exhibits dynamic characteristics indicated by a load line of FIG. 3. In other words, the generation rate of harmonic components increases because this amplifier is configured to amplify only a half of an input sine wave, As a result, there is a problem in terms of linearity. For this reason, such an amplifier cannot be applied to circuits requiring a high linearity.
Therefore, it is required to provide a power amplifier with a high linearity and a high efficiency which is capable of amplifying an input signal without any distortion of the entire wave thereof while obtaining a high gain at a radio frequency, in order to solve the problems involved in conventional push-pull power amplifiers having a source follower configuration and conventional power amplifiers having a common source configuration.